linux-kernel - Re: [PATCH 1/3] tty: n_gsm: fix buffer over-read in gsm_dlci

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Message-ID: <b17dde9a-6519-15be-07c6-218b1a1ef416@kernel.org>
Date:   Mon, 9 May 2022 12:41:17 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     "D. Starke" <daniel.starke@...mens.com>,
        linux-serial@...r.kernel.org, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tty: n_gsm: fix buffer over-read in gsm_dlci_data()

On 04. 05. 22, 10:17, D. Starke wrote:
> From: Daniel Starke <daniel.starke@...mens.com>
> 
> 'len' is decreased after each octet that has its EA bit set to 0, which
> means that the value is encoded with additional octets. However, the final
> octet does not decreases 'len' which results in 'len' being one byte too
> long. A buffer over-read may occur in tty_insert_flip_string() as it tries
> to read one byte more than the passed content size of 'data'.
> Decrease 'len' also for the final octet which has the EA bit set to 1 to
> write the correct number of bytes from the internal receive buffer to the
> virtual tty.
> 
> Fixes: 2e124b4a390c ("TTY: switch tty_flip_buffer_push")

That commit barely introduced the problem.

> Cc: stable@...r.kernel.org
> Signed-off-by: Daniel Starke <daniel.starke@...mens.com>
> ---
>   drivers/tty/n_gsm.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index a38b922bcbc1..9b0b435cf26e 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -1658,6 +1658,7 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
>   			if (len == 0)
>   				return;
>   		}
> +		len--;
>   		slen++;
>   		tty = tty_port_tty_get(port);
>   		if (tty) {


-- 
js
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